Astronomy & Astrophysics

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Issue		A&A Volume 440, Number 3, September IV 2005
Page(s)		881 - 892
Section		Extragalactic astronomy
DOI		10.1051/0004-6361:20052893

A&A 440, 881-892 (2005)
DOI: 10.1051/0004-6361:20052893

Spectroscopy and stellar populations of star-forming galaxies at z ~ 3 in the Hubble Deep Field - South

I. Iwata^{1, 2, 3}, A. K. Inoue^{4, 3, 5} and D. Burgarella⁴

¹ Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, Honjo, Kamogata, Okayama 719-0232, Japan
e-mail: iwata@oao.nao.ac.jp
² Subaru Mitaka Office (Subaru Telescope), National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan
³ Department of Astronomy, Faculty of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan
⁴ Laboratoire d'Astrophysique de Marseille, Traverse du Siphon, BP 8, 13376 Marseille Cedex 12, France
⁵ Department of Physics, Faculty of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan

(Received 17 February 2005 / Accepted 24 May 2005)

Abstract
We present results of VLT/FORS2 spectroscopy of galaxies at $z \sim 3$ in the Hubble Deep Field - South (HDF-S). A sample of galaxies was drawn from the photometric redshift catalogue based on the HST/WFPC2 optical images and the deep near-infrared images obtained with VLT/ISAAC as a part of the Faint Infrared Extragalactic Survey (FIRES) project. We selected galaxies with photometric redshift between 2.5 and 4. Most of the selected galaxies are bright in rest-frame UV wavelengths and satisfy color selection criteria of Lyman break galaxies (LBGs) at $z \sim 3$ . The number of target galaxies with $I_{\rm AB} \leq 25.0$ was 15. We identified new 5 firm and 2 probable redshifts in addition to confirmation of 6 previously known galaxies at $z \sim 3$ . We found that 6 among these 13 galaxies lie in a quite narrow redshift range at $z = 2.80 \pm 0.01$ . Their spatial distribution is fairly concentrated and is at the edge of the HDF-S field, suggesting the possible existence of larger galaxy clustering. We examined stellar populations of the galaxies with spectroscopic redshifts through comparisons of their optical and near-infrared photometry data with template spectra generated by a population synthesis code. The ages from the onset of star formation for these star-forming galaxies with $I \leq 25.0$ are typically 50-200 Myr, and their stellar masses are between $(0.5{-}5) \times 10^~M_{\odot}$ , consistent with previous studies. We also compared these SED fitting results with those for "distant red galaxies" (DRGs) at z > 2 discovered by FIRES. DRGs have larger stellar masses, higer dust attenuation than our UV-luminous LBG sample, and their star formation rates are often comparable to LBGs. These trends suggest that majority of the DRGs are indeed the most massive systems at that redshift and are still in the active star-forming phase. Unless the number density of DRGs is much smaller than LBGs, estimates based on a UV selected sample could miss substantial part of stellar mass density at $z \sim 3$ .

Key words: cosmology: observations -- galaxies: high-redshift -- galaxies: distances and redshifts -- galaxies: evolution -- galaxies: stellar content

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